@DA01 - It is true that physical characteristics are more important, but much of this can generally be summed up by describing the average age at which most children meet those qualifications. Besides, most parents - especially those with school age children that aren't home schooled - are generally familiar with where their child fits in size-wise, when compared to most other kids of a given age, and can apply that knowledge appropriately.
–
IsziOct 31 '11 at 2:41

3

Well, the guidelines are pretty much weight and height. The age can be used as a generic ballpark, but kids at these ages have quite a range of sizes, so it really is best to focus on weight/height.
–
DA01Oct 31 '11 at 4:19

Many states now have laws requiring two of three criteria, I would check the laws in your state regardless of recommendations that come from manufactures and others.
–
ErinNov 3 '11 at 1:26

Thank you! It was a very thorough question, which helped.
–
SaratoOct 31 '11 at 20:30

1

The age a child can move to the front seat can also be governed by local laws. In Australia, the child must be at least 12. My 10 year old daughter is the same height as my wife but is stuck in the back seat for another two years.
–
daveNov 2 '11 at 19:05

Although Sarato has given a good general overview, The answer to this will vary depending on the make and model of the carseats as well as local laws.

For example, some infant carrier carseats support a weight limit of 22 lb, but others have a weight limit of 30 lb. An infant carrier carseat can be used as long as the child is within the height and weight limits of the seat, and the seat fits properly (ie: the shoulder harness goes high enough and their head is not too close to the top of the seat). For some children and seats, it may be only 4 months before they outgrow their infant carseat, others may be able to use one until they are a year or more old.

In the state of California (where I happen to live), the new law going into effect in January is that children must sit in the backseat until they are 12 (with exceptions, such as backseat is already full with additional children), and be in a booster seat or carseat until they are 8.

The NHSTA has a helpful infographic representing the age-appropriateness of the different types of seats. Eg: rear-facing 0-3, forward-facing 1-7, booster seat 4-12, seat belt 8+. Notice that these do not reflect the American Academy of Pediatric's most recent stance that children should be kept rear-facing until at least age 2 or as long as possible (within the limits of the carseat).